1. Field of the Invention
The present patent application relates to a TFA image sensor with stability-optimized photodiode for converting electromagnetic radiation into an intensity-dependent photocurrent with an intermetal dielectric, on which, in the region of the pixel matrix, a lower barrier layer (metal 2) is situated and a conductive layer (metal 2) is situated on said barrier layer, and vias being provided for the contact connection to the ASIC, said vias in metal contacts on the ASIC.
2. Discussion of the Related Art
A TFA sensor (Thin Film on ASIC (TFA) Technology) comprises a matrix-organized or linear arrangement of pixels. The electronic circuits for operating the sensor (e.g. pixel electronics, peripheral electronics, system electronics) are usually realized using CMOS-based silicon technology and form an application specific integrated circuit (ASIC).
Isolated therefrom by an insulating layer and connected thereto by means of corresponding electrical contacts, there is situated on the ASIC a multilayer arrangement as photodiode, which performs the conversion of electromagnetic radiation into an intensity-dependent photocurrent. Said photocurrent is transferred at specific contacts—present in each pixel—of the pixel electronics underneath (B. Schneider, P. Rieve, M. Böhm, Image Sensors in TFA (Thin Film on ASIC) Technology, ed. B. Jähne, H. Hausecker, P. Geiβler, Handbook of Computer Vision and Applications, pp. 237-270, Academic Press, San Diego, 1999).
According to the prior art (J. A. Theil, M. Cao, G. Kooi, G. W. Ray, W. Greene, J. Lin, A J. Budrys, U. Yoon, S. M a, H. Stork, Hydrogenated Amorphous Silicon Photodiode Technology for Advanced CMOS Active Pixel Sensor Imagers, MRS Symposium Proceedings, vol. 609, 2000), what is used as photodiode is a pin configuration based on amorphous silicon, i.e. a sequence comprising a p-conducting, an intrinsically conducting (intrinsic) and an n-conducting amorphous silicon layer. The n-type layer usually forms the bottom most layer facing the ASIC.
The electrical contacts are formed by a metal layer, for example, on said side facing the ASIC, while the contact connection on the side facing the direction of light incidence is generally effected by a transparent and conductive layer.
Over and above the pin photodiode mentioned, further component structures are also possible, e.g. Schottky photodiodes, in which an intrinsic semiconductor layer is brought into contact with a suitable metal (for example chromium, titanium, platinum, palladium, silver), so that the metal-semiconductor junction forms a Schottky photodiode.
A typical layer configuration is disclosed in the patent application TFA image sensor with extremely low dark current, Y, S, Seruak Bi, Ser. No. 10/541,440. Furthermore, detector structures with a controllable spectral sensitivity are known (P. Rieve, M. Sommer, M. Wagner, K. Seibel, M. Böhm, a-Si:H Color Imagers and Colorimetry, Journal of Non-Crystalline Solids, vol. 266-269, pp. 1168-1172, 2000). This basic structure of a TFA image sensor can furthermore be extended by additional, upstream layers in the direction of light incidence, for example by color filter layers (e.g. Bayer pattern, U.S. Pat. No. 3,971,065).
If amorphous silicon is used as photoactive sensor material, then the metastability observed in the case of this material becomes apparent, under certain circumstances. Hydrogenated amorphous silicon (a-Si:H) comprises a silicon-hydrogen atomic composite lacking a long-range order as is typical of semiconductor crystals. Modifications of the atom bonding parameters occur with respect to the ideal semiconductor crystal. The consequence of this is that, in the context of the solid-state band model, a state density that differs from zero exists in the band gap between conduction band and valence band, which affects the electrical and optical properties of the material. States in the middle of the band gap predominantly act as recombination centers, while states in the vicinity of the band edges function as traps for charge carriers. On account of light being radiated in or injection of charge carriers, more precisely through recombination of injected charge carriers, weak silicon bonds are broken and additional band gap states arise.
These band gap states caused by light irradiation represent additional recombination or trapping centers and influence the charge carrier transport and the distribution of the electric field strength in the components fabricated from amorphous silicon. In pin photodiodes, for example, predominantly positively charged states are concentrated in traps in that region of the intrinsic layer (i-type layer) which adjoins the p-type layer, and negatively charged states in that region of the i-type layer which adjoins the n-type layer. These stationary charges result in a decrease in the magnitude of the electric field strength within the i-type layer, so that the accumulation of photogenerated charge carriers deteriorates. An efficient charge carrier accumulation in pin photodiodes made of amorphous silicon is provided when the drift length (μτE) of the charge carriers significantly exceeds the thickness d of the intrinsic layer:μτE>>d  (1)
On account of the increase in the defect density associated with light being radiated in, on the one hand the lifetime τ is reduced due to intensified recombination of charge carriers and on the other hand the electric field E is reduced on account of the charged states in the i-type layer. Both have the consequence that the ratio of drift length to i-type layer thickness is reduced and the photocurrent thus decreases. The decrease in the photocurrent becomes apparent particularly when the photodiode is operated near the short-circuit point without additional reverse voltage, i.e. when only the built-in potential difference brought about by the doped layers is effective.
When reverse voltage is applied, by contrast, the electric field intensifies, so that the charge carrier accumulation is impaired to a less extent. The essential consequence of the light irradiation of a photodiode made of amorphous silicon with regard to the photocurrent thus consists in a reduction of the photocurrent saturation.
The dark current of an a-Si:H photodiode, i.e. the current which flows even in the unilluminated state is likewise influenced by the degradation of the material. On account of the defect states additionally generated by light being radiated in, the thermal generation of charge carriers increases in the case of a reverse-biased photodiode (extraction), which is manifested in an increase in the dark current.